The present invention relates to a process for calcining limestone for producing quicklime or quicklime-like products by supplying heat into a kiln carrying the limestone, in which lean gas is produced from waste energy carriers, whereby said gas subsequently undergoes dust removal, accompanied by the extraction of the necessary calcining heat in the limestone-carrying kiln, where it is burned, the combustion waste gases being conveyed through the limestone bed. The invention also relates to an installation for performing the process for calcining limestone with a lime kiln, in which the limestone is passed into a hopper and brought into contact with calcining heat produced in burners arranged in distributed manner and which is followed by a dust catcher.
Limestone is calcined in large lime kilns, in that a temperature of approximately 1000.degree. C. is produced in the kiln by conventional energy carriers such as coal, oil and natural gas. The energy carrier must be sought as a function of the quality of the lime to be produced, because otherwise the introduction of dust leads to a quality reduction. The general quality requirements made on the product quicklime, no matter whether it is hard or soft burned, do not allow an increased introduction of dust into the calcining part of the kiln, because this has a negative influence on the quality and colour of the product. Thus, despite the relatively high energy costs for this kiln process, it has hitherto been standard practise to stay with natural gas and oil, and less frequently coal. Waste energy carriers, such as fuel obtained from refuse, old tires, car shredding refuse, plastic waste and also used oil have a relatively high ash proportion compared with their calorific value. In the case of combustion in the conventional lime kiln, such ash would lead to the aforementioned significant reduction in the quality of the end product. Therefore, it has hitherto been considered impossible to use such waste energy carriers for calcining limestone.
DE-OS-26 47 021 discloses a process for operating a lime shaft kiln with a furnace or kiln chamber located outside the same, in which, under air deficiency, old tires are thermally decomposed, the decomposition products are introduced into the burning zone of the shaft kiln and after burned therein. The decomposition products are suppled from the furnace chamber of the shaft kiln or furnace via one or more ring mains into one or more planes. The old tires are introduced into the furnace chamber in comminuted form and are exposed to a temperature of over 1000.degree. C. for a relatively short period. However, this known process is not suitable for gasifying waste materials having a lower calorific value than old tires, or comprising a heterogeneous mixture of different waste material types. Gasification takes place below the melting point of the solid gasification residues. Any harmful or polluting substances occurring during the gasification, such as dust containing dioxin or the like remain bound and must be eliminated in a complicated and costly manner.
The problem of the present invention is therefore to provide a process and an installation of the aforementioned type, in which it is possible to avoid during the gasification of the waste energy carriers dust containing polluting substances in unbound form.